Words like おむつ (diaper) and おつむ (head) apparently came from slang used by female attendants of the imperial court (宮中きゅうちゅう女官にょかん).

The slang came from taking the first two mora of a word and adding the prefix お. Most of these are now in common use in modern Japanese.


Original Slang Meaning
むつき おむつ diaper (orig. swaddling cloth)
つむり おつむ head (of baby)
らし おなら fart
やし おひや cold water
田楽でんがく おでん oden (fish cake stew)
寝小便ねしょうべん おねしょ bed-wetting
数々かずかず おかず side dish to rice (from かぞえる)